It may be possible to confirm family relationships by comparing test results with Douglas or other carriers of his ancestors' Y-chromosome or mitochondrial DNA.
Y-chromosome DNA test-takers in his direct paternal line on WikiTree:
Family Tree DNA Y-DNA Test 67 markers, haplogroup M-201, FTDNA kit #204988
Mitochondrial DNA test-takers in the direct maternal line:
Family Tree DNA mtDNA Test HVR1 and HVR2, haplogroup T, FTDNA kit #204988